A mainstream light source on the market is of a design that a prism sheet (P) or the like is, as shown in FIG. 27, combined with a sidelight type light guide plate (L) having a diffuse reflection plate (R′) disposed below the light guide plate to thereby increase an incident component in the front direction. In the light source of FIG. 27, a diffusing plate (Y) is disposed above the prism sheet. Generally, a light emitting angle of a sidelight type light guide plate is an angle shallow relative to the upper side or lower side and of 60° or more from the front direction and light emitting angles are, in more of cases, distributed within a range of shallow angles of 70° or more from the front direction. Since a light guide plate of this kind guides light in a resin plate using a critical angle and light is taken out in a state outside the critical condition, lights emitting at a shallow angle in an oblique direction are predominant as compared with others. Hence, a prism sheet is generally used as a contrivance to condense light in the front direction.
Such a light source is rare in design that more of normally emitting light component is obtained, for an essential reason originating from the structure as described above. A manufacturing cost has been, in more of cases, high because of its structure. A combination of a light guide plate and a prism sheet generally available in the market renders the prism sheet easily scratched and problematic in handling since the prism sheet is disposed on the upper surface side of the light guide plate. Consequently, such a light source has been exposed to a strong demand for improvement on handlability.
On the other hand, a technique is known that an optical layer (T) having an angle dependence of transmittance and reflectance such as a vapor deposited band pass filter using a Brewster angle and a layer of a cholesteric liquid crystal using a Bragg reflection, and having a selective reflection characteristic is adopted to thereby condense a diffuse light source in the front direction. A light source using such a technique is shown in FIG. 28. Proposed as a typical example is a method in which a bright-line light source is combined with a band-pass filter (see, for example, Japanese Patent Application-LAID-OPEN Nos. 6-235900, 2-158289 and 10-321025, U.S. Pat. No. 6,307,604, DE 3836955 A and DE 4222028 A, EP 578302 A, US 2002-34009 A, WO 02/2568 A1). Proposed is a method in which a light source emitting a bright line such as CRT and electroluminescence and a band-pass filter on a display are arranged to condense and collimate light beams (see, for example, Japanese Patent Application-LAID-OPEN Nos. 2001-521643 and 2001-516066 and, US 2002-036735 A, Japanese Patent Application-LAID-OPEN No. 2002-90535 and others).
Though normally incident light can be used at a high transmittance in a case where an optical layer having an angle dependence of a transmittance and a reflectance is employed, obliquely incident light is reflected thereon and not transmitted therethrough; therefore, the obliquely incident light is returned back to the light source side. The returned back light beam is transmitted through or reflected back on a light guide plate, a scattering plate and a reflection plate and then, again returned back to the emitting side, during which absorption loss is not small because of repetition of reflection and others. A liquid display using a reflection polarizer as a light source, which has been generally used, can be theoretically expected to have two hold brightness, but an actual brightness is limited to a value on the order of 1.5-hold and the balance is wasted as an absorption loss, which naturally cannot be used.
In order to enhance a recycling efficiency, an idea comes to mind that light from a light source is returned to the light emitting side by decreasing a chance of a loss due to reflection, scattering and transmission to the lowest possible level and selecting the shortest possible optical path. For example, if a light guide plate rich in normally emitting light component is adopted as a light guide plate combined with the transmittance angle dependent layer, a utilization efficiency of emitting light is improved. In a transmittance angle dependent layer, however, no consideration is given to an optical path of a reflecting light component to be recycled and as the number of times of repetitive reflections increases, emitting light from a light source makes useless stray light, which naturally results in absorption loss without being used. Besides, a light guide plate combined with a prism sheet has had the problem. Hence, options for a light guide plate combined with a transmittance angle dependent layer have been extremely limited.